Due to the characteristics of thin profile and low power consumption, liquid crystal displays (LCDs) are widely used in electronic products, such as portable personal computers, digital cameras, projectors, and the like. Generally, LCD panels are classified into transmissive, reflective, and transflective types. A transmissive LCD panel uses a back-light module as its light source. A reflective LCD panel uses ambient light as its light source. A transflective LCD panel makes use of both the back-light source and ambient light.
As known in the art, a color LCD panel 1 has a two-dimensional array of pixels 10, as shown in FIG. 1. Each of the pixels comprises a plurality of sub-pixels, usually in three primary colors of red (R), green (G) and blue (B). These RGB color components can be achieved by using respective color filters. FIG. 2 illustrates a plan view of the pixel structure in a conventional transflective liquid crystal panel, and FIG. 3 is a cross sectional view of the pixel structure. As shown in FIG. 2, a pixel can be divided into three sub-pixels 100R, 100G and 100B and each sub-pixel can be divided into a transmission area (TA) and a reflection area (RA).
A typical sub-pixel 100 is shown in FIG. 3. As shown, the sub-pixel segment 100 has an upper layer structure, a lower layer structure and a liquid crystal layer 190 disposed between the upper layer structure and the lower layer structure. The upper layer comprises a polarizer 120, a haft-wave plate 130, a quarter-wave plate 140, a color filter (not shown) and an upper electrode 150. The upper electrode 150 is made from a substantially transparent material such as ITO (Indium-tin oxide). The lower layer structure comprises an electrode layer having a transmissive electrode 170 and a reflective electrode 160. The transmissive electrode 170 is made from a transparent material such as ITO. The reflective electrode 160 also serves as a reflector and is made from one or more highly reflective metals such as Al, Ag, Cr, Mo, Ti, and AlNd. The lower layer structure further comprises a passivation layer (PL) 180, a device layer 200, a quarter-wave plate 142, a half-wave plate 132 and a polarizer 122. In addition, the transmissive electrode 170 is electrically connected to the device layer 200 via a connector 184, and the reflective electrode 160 is electrically connected to the device layer 200 via a connector 182.
In the transmission area as shown in FIG. 3, light from a back-light source enters the pixel area through the lower layer structure, and goes through a liquid crystal layer and the upper layer structure. In the reflection area, light encountering the reflection area goes through the upper layer structure and the liquid crystal layer before it is reflected by a reflective electrode 160.
The sub-pixel structure as shown in FIG. 3 is known as a single-gap structure. In a single-gap transflective LCD, one of the major disadvantages is that transmittance of the transmission area (the V-T curve) and reflectance in the reflection area (the V-R curve) do not reach their peak values in the same voltage range. As a result, reflectance experiences an inversion while transmittance is approaching its higher value.
In order to overcome this inversion problem, a dual-gap design is used in a transflective LCD. In a dual-gap transflective LCD, as shown in FIG. 4, the gap GR in the reflection area is half the gap GT in the transmission area. Thus, the thickness of the liquid crystal layer 190 in the reflection area is one half the thickness of the liquid layer 190 in the transmission area. As such, the transmittance and the reflectance of the LCD are more consistent with each other.
A normally white modulated twisted nematic LCD is shown in FIGS. 5A and 5B. As shown in FIG. 5A, when the applied voltage across the liquid crystal layer 190 is low, the liquid crystal molecules are aligned along with the surface of the upper electrode 150 such that the liquid crystal layer 190 acts like a half-wave plate in both the transmission area and the reflection area. When the applied voltage across the liquid crystal layer 190 is high, the liquid crystal layer no longer acts as a half-wave plate. The liquid crystal layer 190 does not change the polarization state of a light beam traversing the layer, as show in FIG. 5B.
While the optical characteristics of a dual-gap transflective LCD are superior to those of a single-gap transflective LCD, the manufacturing process for controlling the gap in the reflection area in relation to the gap in the transmission area is complex. The production yield for dual-gap transflective LCDs is generally lower than that of single-gap transflective LCDs.
It is thus desirable and advantageous to provide a method for achieving a transflective LCD having the advantages of both the single-gap transflective LCD and the dual-gap transflective LCD.